1. Field of the Invention
The present invention relates generally to a system for transmitting and receiving data in a mobile communication system and in particular, to a system for transmitting and receiving data in a mobile communication system utilizing optical satellite technology.
2. Description of the Related Art
Mobile communications are rapidly replacing fixed communications systems as today's users are becoming more accustomed to enjoying the freedom of accessing voice and data information anyplace and at anytime. The growth is occurring at such a rapid pace that the present cellular communications systems are quickly reaching their capacities, resulting in communication companies scrambling for more bandwidth and general expansion of their existing systems. Increasing bandwidth and capacity of the current systems has become first priority for many communication companies. As quickly as the bandwidth and capacity is increased, just as quickly it is utilized by the ever-increasing demand of the users. Basic cellular networks also require numerous cell-sites that require cellular antenna towers to be erected every few miles to provide adequate coverage. The total number of cell sites in the U.S. alone numbers in the hundreds of thousands. In addition, even with the great number of cell sites, coverage is still not guaranteed to all areas, as costs associated with placing cell sites in remote locations is not commercially economical.
Another drawback with cellular systems is the requirement for multiple protocols required to route calls throughout the system. Extensive conversion algorithms are required to accomplish the protocol based routings.
A major breakthrough occurred in increasing the bandwidth by utilizing fiber optic technologies in communication systems. The fiber optic networks economically interconnect higher density urban areas. Although fiber optics was also found to be quite useful in crossing oceans, repair and replacement of damaged cables is a costly and constant concern. In addition, the fiber optic networks are not readily available in remote regions of the earth due to geographic and economic constraints. For example, it is not economically viable or geographically feasible to construct a fiber optic network to the middle of the Sahara Desert or to the top of Mount Everest. Also fiber optic technology has the inherent drawback of being a fixed system, unable to provide voice and data communication services to users in moving vehicles, whether land, sea or airborne.
In an attempt to provide voice and data communications to remote and inaccessible areas of the planet, satellite technologies have been introduced. Many of the satellite based communications systems are radio frequency (RF) based systems. These RF systems receive and transmit signals to and from mobile or stationary locations. The RF satellite systems require high power to transmit signals from the satellites and user equipment (UE). Additionally, by the nature of limited RF frequencies available to the communications systems, the RF systems are an expensive option to use as the number of users per satellite is not cost effective. Also, cloud cover or even high humidity can often interfere with and even prevent useful communications.
In an attempt to expand the capacity of the satellite systems, use of optical link transmissions to and from the satellites is currently being developed. Generally being utilized in these developments are laser link transmissions. Although this technology has been theorized since the introduction of the laser in the 1960s, a viable system has yet to be developed. Many individuals and groups are experimenting with various systems that include low earth orbit (LEO), medium earth orbit (MEO), and geosynchronous orbit (GEO) satellites for optical linkage with ground and airborne stations. In addition to the very high bit rates of an optical based system, the power requirements of an optical system are greatly reduced compared to that of the RF systems, thus increasing their theoretical consumer value.
The above-mentioned optical satellite systems also require extensive pointing, acquiring and tracking systems. An optical communication satellite must first be instructed to point its optical transmitter and receiver in the direction of a fixed or mobile target station. This is usually accomplished utilizing an RF control signal that is sent to the satellite. After the optics of the satellite is aligned in a proper direction, the satellite must acquire the target station signal. As a method of performing the acquisition, a laser in the satellite could perform a search similar to that of early radar conical scan searching, wherein the satellite defocuses a laser beam and begins to focus the beam making micro-radian mechanical gimbal adjustments to search for stronger portions of the signal as it tightens the pattern. After alignment and acquisition, the satellite needs to perform a continuous tracking procedure to maintain a proper communication link during the transfer of the voice and data information. A typical laser tracking system would require optics, electronics and mechanical hardware to continually monitor and adjust to the movements associated with the satellite and target station or user equipment. After a point-to-point link is established, sustained communication can occur. The electronic and mechanical systems to perform these procedures are costly, and increase greatly the size and weight of the satellite.
These current optical satellite systems are quite limited in their total user capabilities, and the tracking and switching in these point-to-point systems greatly increase their cost and size. Even with the theorized optical systems, the cost to utilize these optical satellite systems to the average user would be astronomical. Also, to achieve total coverage of a vast area, for example the U.S., the present satellite systems would require massive amounts of satellites.
As stated above with respect to the satellite systems, although the coverage area is increased, the cost per user is currently at a premium. In addition, the present satellite networks are again constrained by their bandwidth capacities. Attempts to expand the bandwidth of the existing satellite networks are proving quite expensive and not quite feasible. Although tracking of mobile UE by satellites is feasible and currently in use, the tracking capability of each satellite is limited to only a small number of mobile locations at any one time.
A further problem with existing laser based satellite systems is a degradation of the optical signal that occurs as a result of atmospheric diffusion. As a laser is projected through the atmosphere, the signal is constantly being diffused by particles in the air. These particles include water particles, smog, and cloud cover, to name a few. Ideally, a laser-based system can only operate at an optimum level in arid and clear locations, thus limiting its usefulness in more needed locations.
There is therefore a need to provide a voice and data communication system that has nearly unlimited bandwidth capabilities and can provide voice and data communications to nearly unlimited multiple mobile users, whether the user is on land, sea or in air.